how is the refund amount calculated for auto-renewing upgrade?

Hi, We have a app with some auto-renewing subscription in a group of subscriptions.

When a user upgrade from a subscription to another, the "user receive a refund of the prorated amount of their original subscription" (https://developer.apple.com/app-store/subscriptions/).

How is the prorated calculated ?

Example : subscription to 14,99$ / month. If subscriber upgrade after 10 days, is the refund calculated 10/30 of 14,99$ (so ~5$) ?

how is the refund amount calculated for auto-renewing upgrade?
 
 
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